In the late 20's, hillclimbing was gaining popularity across the US. The only problem was Harley was getting beaten by both Indian and Excelsior. In an effort to beat the competition, Harley developed the DAH motor. Only about 25 were made, but thanks to some advanced features like overhead valves and recirculating oil, the DAH brought Harley hillclimb victories throughout the 1930's.
The DAH Harley-Davidson Factory Racer
The DAH Harley-Davidson Factory Racer